I frequently recommend to folks who think they might be interested in a Yorkie that they go to a dog show and see them in the ring, watch them interact with their handler and the other dogs.
Potty training is a consistency issue on the part of the human it seems. Like LovesToDream said.
My Yorkie was a show dog and it was decided that she would be too small to breed to get the next champion so I got her. Showing creates a pup with a lot of discipline and well socialized, so she is not happy or hyper. She is however very much in charge and sassy! And yes, she is a hunter, a 4 pound guard dog, very alert, and just about perfect
Their human people are VERY important to them!